The mixing of rubber composition conventionally is done on roll mills or in a Banbury internal mixer. When using roll mills for mixing the rubber composition, the rubber component of the composition is banded on the roll mill and the remaining compounding ingredients are added to the rubber as it is being kneaded on the roll mills. When mixing of a rubber composition is carried out in a Banbury internal mixer, the rubber component of the composition is added to the mixer and the remaining ingredients then are added to the mixer. Mixing continues on the roll mills or within the Banbury internal mixer until the compounding ingredients are substantially uniformly dispersed throughout the rubber matrix.
Sulfur or a sulfur-donor is included in sulfur-vulcanizable rubber compositions to produce chemical cross-linking between rubber polymer chains to form the polymer chains into a stereo-network. The crosslinked rubber polymer product exhibits improved toughness and durability. The rate at which crosslinking occurs (the "cure rate") is extremely slow at room temperature but is increased significantly if the composition is heated. The rate at which crosslinking occurs also can be improved through the use of accelerators of vulcanization that are added to the rubber composition in combination with sulfur on sulfur-donor materials. Further improvement in the cure rate can be achieved through the use of accelerator activators which form chemical complexes with the accelerators that not only benefit the rate at which the rubber composition cures but also may improve the physical or chemical properties of the vulcanized composition.
During the mixing operation, whether mixing of the rubber composition is accomplished on roll mills or in a Banbury internal mixer, a significant heat build-up in the composition is experienced as a result of friction developed during the kneading of the rubber compound of the composition. Prolonged mixing of the ingredients at the elevated temperature will result in "scorching" the composition. ("Scorch" is a premature partial or complete vulcanization of the rubber composition during the mixing operation.) A premature vulcanization of the batch during the mixing operation deminishes the plastic properties of the composition so that it can no longer be processed satisfactorily and results in the batch having to be discarded. Vulcanization of the batch within a Banbury internal mixer not only results in a ruined batch but also often requires a costly clean-up operation to remove the vulcanized batch from the interior of the mixer.
As a consequence of the "scorching" problem, care must be exercised in formulating a rubber composition to avoid "scorching". If accelerators and accelerator activators are included in the composition in order to reduce the cure rate of the composition or for producing desired physical or chemical properties in the vulcanized product, scorching of the batch during mixing is risked. To avoid the risk of scorching, rubber compounders have materially reduced or in some instances entirely eliminated accelerators or accelerator activators from certain formulations, even though to do so resulted in an objectionably long time to reach an optimum vulcanization of the composition and a sacrifice in the physical or chemical properties of the vulcanized product.
In the copending application of Stephen C. Sabo, James Sidles and Jerome J. Blayne entitled "Process For Marking Rubber Articles", Ser. No. 145,167, filed Apr. 30, 1980, a process is described for mixing a rubber composition that normally would have "scorched" if mixed by conventional mixing procedures using roll mills or a Banbury internal mixer. The process involves forming two or more strips of rubber compositions of different formulations. One strip contains the sulfur or a sulfur-donor while the other strip contains an accelerator system for reducing the cure time necessary to fully vulcanize the composition. The strips are fed at essentially an equal rate into an extruder in which the strips are intermingled as the extruder screw carries the compositions toward the extruder discharge head. The intermingling of the separate compositions as they travel through the extruder produces an extrudate that consists of relatively thin juxtapositioned layers or strata of the separate compositions. Migration of the sulfur or sulfur-donor from one stratum to adjacent strata and migration of the accelerator system from one stratum to adjacent strata occurs whereby the extrudate progresses toward a condition of equilibrium equivalent to that which would have resulted if the sulfur or sulfur-donor and accelerator system had been added initially with the rubber polymer of the composition on roll mills or in a Banbury internal mixer but without the danger of scorching.
To achieve an optimum volume mix of the separate strips of rubber composition, the strips must be fed simultaneously into the extruder at substantially equal linear rates. However, if the strips merely are fed into the extruder, relying solely on the extruder screw to draw the separate strips of rubber composition into the extruder, the strips normally are pulled into the extruder at significantly different rates. The present invention provides a feeder device that insures the separate strips of rubber composition are fed into the extruder at substantially the same linear rate to produce an optimum volume mix of the two compositions.